One way to get multiclass working is like this using -selectds and settings below
https://docs.google.com/document/d/1BP2cupcjoCdx6pDJfLCIawGhsv_Sjh8O8kKlC_larS4/edit
     controlGameSetup : false
    "ServerControlsTrack" : 0,
    "ServerControlsVehicleClass" : 0,
    "ServerControlsVehicle" : 0,


Like this is not working as it should be at moment.
 if VehicleClassId is uncommented as it tells you to do for multiclass it restricts to that class only.
so i use like this till a fix comes you get the odd 4th class but it works. You will get Ai on first entry to server but gone till next restart.
     controlGameSetup : true
    "ServerControlsTrack" : 1,
    "ServerControlsVehicleClass" : 1,
    "ServerControlsVehicle" : 0,

     // "VehicleClassId" : "LMP1", (if you use this line it restricts to that class so i leave it commented )
        "MultiClassSlots" : 3,
   "MultiClassSlot1" : "GTE", //1740243009,
   "MultiClassSlot2" : "GT3", //-112887377,
   "MultiClassSlot3" : "GT4", //1553262379,

gladly talk you through it.
 once the rotation setup it is pretty easy to change track and cars in the rotation single class work fine like this too
 I will zip my server up and send you a link
It it setup as this first page below

http://forum.projectcarsgame.com/showthread.php?56669-Dedicated-Server-Configuration-Sample-FIXED-Time-Weather-MultiClass!-11-2-17